Index: include/clang/AST/ASTImporter.h
===================================================================
--- include/clang/AST/ASTImporter.h
+++ include/clang/AST/ASTImporter.h
@@ -198,7 +198,7 @@
     /// Return the copy of the given declaration in the "to" context if
     /// it has already been imported from the "from" context.  Otherwise return
     /// NULL.
-    Decl *GetAlreadyImportedOrNull(Decl *FromD);
+    Decl *GetAlreadyImportedOrNull(const Decl *FromD) const;
 
     /// Import the given declaration context from the "from"
     /// AST context into the "to" AST context.
Index: lib/AST/ASTImporter.cpp
===================================================================
--- lib/AST/ASTImporter.cpp
+++ lib/AST/ASTImporter.cpp
@@ -1575,6 +1575,9 @@
     return Err;
 
   ToD = cast_or_null<NamedDecl>(Importer.GetAlreadyImportedOrNull(D));
+  if (ToD)
+    if (Error Err = ASTNodeImporter(*this).ImportDefinitionIfNeeded(D, ToD))
+      return Err;
 
   return Error::success();
 }
@@ -7702,13 +7705,10 @@
   return ToAttr;
 }
 
-Decl *ASTImporter::GetAlreadyImportedOrNull(Decl *FromD) {
-  llvm::DenseMap<Decl *, Decl *>::iterator Pos = ImportedDecls.find(FromD);
+Decl *ASTImporter::GetAlreadyImportedOrNull(const Decl *FromD) const {
+  auto Pos = ImportedDecls.find(FromD);
   if (Pos != ImportedDecls.end()) {
     Decl *ToD = Pos->second;
-    // FIXME: move this call to ImportDeclParts().
-    if (Error Err = ASTNodeImporter(*this).ImportDefinitionIfNeeded(FromD, ToD))
-      llvm::consumeError(std::move(Err));
     return ToD;
   } else {
     return nullptr;
Index: lib/AST/ExternalASTMerger.cpp
===================================================================
--- lib/AST/ExternalASTMerger.cpp
+++ lib/AST/ExternalASTMerger.cpp
@@ -144,14 +144,14 @@
     }
     if (auto *ToTag = dyn_cast<TagDecl>(To)) {
       ToTag->setHasExternalLexicalStorage();
-      ToTag->setMustBuildLookupTable();
+      ToTag->getPrimaryContext()->setMustBuildLookupTable();
       assert(Parent.CanComplete(ToTag));
     } else if (auto *ToNamespace = dyn_cast<NamespaceDecl>(To)) {
       ToNamespace->setHasExternalVisibleStorage();
       assert(Parent.CanComplete(ToNamespace));
     } else if (auto *ToContainer = dyn_cast<ObjCContainerDecl>(To)) {
       ToContainer->setHasExternalLexicalStorage();
-      ToContainer->setMustBuildLookupTable();
+      ToContainer->getPrimaryContext()->setMustBuildLookupTable();
       assert(Parent.CanComplete(ToContainer));
     }
     return To;